The present invention generally relates to antennas comprising an array of radiating elements, and methods for exciting the array elements in a manner that exploits the mutual coupling effects between the elements. More particularly, the present invention relates to systems and methods for providing differential-mode excitation of microstrip patch antennas and monolithic microwave integrated circuit (MMIC) antenna arrays, wherein radiation is generated and emitted from substantially the entire top surfaces of the patches, rather than merely from their edges, thereby enhancing the radiation and improving efficiency. Differential-mode excitation schemes according to the invention may be used for, e.g., electronically steering a radiating beam, shaping a radiating beam, and optimizing the gain of the antenna array in a specified direction.
Microstrip antennas (or patch antennas) provide low-profile antenna configurations for applications that require small size and weight. Such antennas are also desirable when there is a need to conform to the shape of the supporting structure, both planar and nonplanar, such as for an aircraft""s aerodynamic profile. These antennas are simple and inexpensive to manufacture using printed-circuit technology, wherein metallic patches (or patch radiators) are typically photoetched onto a dielectric substrate.
The conventional wisdom regarding microwave patch antennas is that the patches radiate from their edges. More specifically, when the elements of a patch antenna array are excited in common mode (i.e., with equal voltages), the fields that are generated are primarily confined to the dielectric space under each surface element, except for the fringing fields at the edges of the elements. The commonly held view of the mechanism of radiation by patch antennas is that it is the fringing fields at the edges that radiate into the air. Indeed, various models and theoretical analyses have been developed to explain this radiation mechanism, such as the slot radiation model (see, e.g., R. E. Munson, xe2x80x9cConformal microstrip antennas and microstrip phase arrays,xe2x80x9d IEEE Trans. Antennas Propagat., vol. 22, pp 74-78. January, 1974) or the cavity model (see, e.g., Thouroude et al, xe2x80x9cCAD-oriented cavity model for rectangular patches,xe2x80x9d Elect. Lett., vol. 26. pp. 842-844, June 1990). Both the slot and cavity models assume radiation comes only from the edges. Other models known to those skilled in the art, including, for example, conformal mapping, moment methods, and Green""s functions, have been developed, which implicitly include fields that are not at the edges. However, these methods offer limited insight into the radiation mechanism.
FIG. 1 illustrates a typical patch antenna array 10 that comprises small conducting surfaces 18 separated from a large parallel ground plane 14 by a dielectric substrate 16. When the same real or complex (real and imaginary or amplitude and phase) RF voltage Vo is applied to each surface 18, an electric field pattern 15 is set up in the dielectric, essentially acting as a capacitor but with a relatively weak fringing fields 12 at the edges (for clarity, fields 12 are not shown continuing into the substrate). The roughly uniform fields 15 under the surface are fairly well shielded from the outside space, but the fringing field at the edges can act as radiating elements. To take advantage of the edge radiators, it may be necessary to excite the capacitive structure in a higher-order mode and using off-center feeds, to avoid mutual cancellation of the radiation from different edges.
Microstrip patch antennas commonly exhibit disadvantageous operational characteristics such as low efficiency, low power, narrow bandwidth, and poor scanning performance. Further, patch antennas are typically excited in an asymmetric manner to generate high-order modes of the dielectric substrate, which adds to the complexity of the electrical feed circuitry.
A natural phenomenon referred to as xe2x80x9cmutual couplingxe2x80x9d occurs when the patches of an antenna array are subjected to differential-mode excitation (e.g., different voltage amplitudes and phases). In particular, when the applied voltages at two or more patches are different, fields will be set up not only within the substrate directly under each patch, but also in the air space above the patches, emanating from one patch and ending on another.
Conventionally, designers of patch antennas ignore or attempt to reduce the effects of mutual coupling. However, it would be highly beneficial to develop a framework for differential-mode excitation of an antenna array that would exploit the mutual coupling between patches to provide efficient radiation from the exposed top surfaces of antenna patches to, thereby, overcome the above noted deficiencies and disadvantages of conventional patch antenna schemes.
The present invention is generally directed to antennas comprising an array of radiating elements, and methods for exciting the array elements in a manner that exploits the mutual coupling effects between the elements. More particularly, the present invention relates to systems and methods for providing differential-mode excitation of microstrip patch antennas and monolithic microwave integrated circuit (MMIC) antenna arrays. It is an objective of the present invention to devise and prescribe differential-mode excitation methods, which impose different radio frequency (RF) voltages or currents at the different array elements (e.g., patches), to thereby generate and emit radiation from substantially the entire top surfaces of the patches, rather than merely from their edges, thereby enhancing the radiation and improving efficiency. Indeed, differential-mode excitation methods according to the invention are employed to operate an antenna array in a manner that exploits the particular susceptibility of array elements to mutual coupling effects such that the array radiates copiously from the top surfaces of the patches instead of merely from their edges.
Various methods according to the invention are provided for generating optimal differential-mode voltages or currents that are applied to elements of an array to thereby achieve particular radiation characteristics. For example, differential-mode excitation schemes enable electronic steering of a radiating beam, shaping of a radiating beam, and optimizing the gain of the antenna array in a specified direction.
In one aspect of the invention, an antenna system comprises an array of radiating elements, voltage generating system (e.g., computer-based systems) for generating differential-mode voltages or currents for exciting the radiating elements, and a device for feeding the differential-mode voltages or currents to the radiating elements, wherein when the differential-mode voltages or currents are applied to the radiating elements, a radiation beam is generated from mutual coupling between the radiating elements in the array.
In another aspect of the invention, a computer is employed to generate a stream of complex numbers (which represent the excitation voltages or currents) that are determined using a radiation model that provides an efficient, yet accurate, model for determining a radiation pattern emitted from an antenna array operating in differential mode. Optimal excitation voltages or currents can be determined to achieve one of possible objectives, such as aiming or steering a radiating beam or optimizing the gain.
In another aspect, various devices and methods are provided for feeding the excitation RF voltages or currents addressed to each radiating element individually, with amplitudes and phases prescribed by the determined complex numbers. Steering of the radiated beam is accomplished by repeatedly issuing new lists of complex numbers to be applied as voltages or currents to the patches.
These and other aspects, objects, features and advantages of the present invention will be described or become apparent from the following detailed description of preferred embodiments, which is to be read in connection with the accompanying drawings.